In the molding under pressure of resin bonded fillers such as wood particles, it is desirable that the amount of compaction should generally be uniform in the final molding process even though the shape of the article may be more or less complicated and the thickness of various parts of the article may vary. Furthermore, if the density of some portion of the article to be made needs to be greater or less than the remainder, that is likely to be a requirement quite independent of the depth of the corresponding location in the mold half in which the molding mixture is placed prior to the final pressing step.
This is particularly important for the molding of structural members of buildings, since they are required to bear loads. The strength of the various portions of the molded articles must be made to stringent specification.
Efforts have been made to meet this problem by filling different portions of the lower mold half to different levels with a molding mixture in accordance with instructions carefully observed by personnel in charge of filling the molds. Distribution of the molding mixture in the lower mold half by vibrating the latter has also been used. Knife blades have also been proposed for profiling the filling of a molding mixture in a mold by scraping the filling down in some places and piling it up in others as the knife blade travels across a filled mold. These methods are not fully suitable for distributing the molding mixture for molding building components of complicated shape, except in special cases usually involving relatively simpler shapes.
In the molding of smaller articles preforms have first been made, by a preliminary molding step or by a subdivision of partly consolidated material, and then subjected to the final load. The provision of preforms large enough for structural members and having sufficient cohesion for some handling without going too far in the compaction of the material or the curing of the binder presents a good many problems.